In general, a system for manufacturing semiconductor devices is structured by combining processing apparatuses of various kinds. A transfer mechanism is arranged to automatically transfer wafers between the processing apparatuses, and between the processing apparatuses and a cassette for storing a number of wafers. For example, the transfer mechanism has a transfer arm, which is extensible/contractible, swingable, and movable up and down. The transfer arm is horizontally moved to a delivery position to transfer and deliver a wafer to a predetermined position. Examples of such a system structured by combining processing apparatuses of various kinds are disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publications Nos. 7-193112, 9-252039, and 2000-127069.
FIG. 7 is a structural diagram schematically showing a processing system of this kind. As shown in FIG. 7, the processing system has a laterally long box-like common transfer chamber 2. A plurality of, e.g., three in this example, waiting ports 4A, 4B, and 4C are disposed on one side along the longitudinal direction of the common transfer chamber 2. Each of the waiting ports 4A, 4B, and 4C is structured to place one cassette container 6 therein on standby. The cassette container 6 can accommodate a plurality of, e.g., 25, unprocessed wafers W with intervals therebetween in the vertical direction.
A plurality of processing apparatuses, e.g., processing apparatuses 10A and 10B in this example, having load-lock chambers 8A and 8B, which can be vacuum-exhausted, are disposed on the side opposite the waiting ports 4A to 4C. For example, the processing apparatuses 10A and 10B are arranged to perform different processes. In each of the load-lock chambers 8A and 8B, two buffer tables 12A or 12B and an individual transfer mechanism 14A or 14B are disposed, so that wafers can be transferred between the processing apparatus 10A or 10B and common transfer chamber 2.
An orientor 16 is disposed at one end of the common transfer chamber 2 to correct a displacement of a wafer, thereby performing alignment of the wafer. A common transfer mechanism 18 is disposed in the common transfer chamber 2 to be movable in its longitudinal direction (X direction). The common transfer mechanism 18 has two transfer arms 20A and 20B, which can be independently controlled to extend/contract, swing, and move up and down. The two transfer arms 20A and 20B are used to perform wafer replacement.
In the processing system described above, an unprocessed wafer W is first transferred from the cassette container 6 in one of the waiting ports 4A to 4C to the orientor 16 by one of the transfer arms, e.g., the arm 20A, of the common transfer mechanism 18. In the orientor 16, the other empty transfer arm, e.g., the arm 20B, is used to pick up a wafer already aligned, so as to replace it with the unprocessed wafer W. The wafer already aligned is transferred to the load-lock chamber of a predetermined processing apparatus, e.g., the load-lock chamber 8A of the apparatus 10A, for performing a necessary process.
A wafer processed in the processing apparatus 10A is transferred into the load-lock chamber 8A. The processed wafer is replaced with the unprocessed wafer having been transferred, as described above, in the load-lock chamber 8A. The unprocessed wafer is transferred into the processing apparatus 10A to be subject to a predetermined process. The processed wafer is returned into the original cassette container 6 by the common transfer mechanism 18.
In such a processing system, it is generally preferable that an unprocessed wafer is placed on standby closely to the processing apparatus 10A or 10B as far as possible, in light of improvement in throughput. Accordingly, unprocessed wafers are sequentially transferred from the cassette container 6, and are respectively placed on standby in the orientor 16, on one of the transfer arms of the common transfer mechanism 18 (the other transfer arm is empty for wafer replacement), and on one of the buffer tables 12A closer to the processing apparatus 10A in the load-lock chamber 8A. When the process on one wafer is complete, the processed wafer is immediately returned into the cassette container. On the other hand, the standby unprocessed wafers are sent in order, so that the next unprocessed wafer is transferred into the processing step.
In this situation, it is assumed that, for example, a cassette container storing new unprocessed wafers is placed in an empty waiting port, e.g., 4C, in order to subject them to the process in another processing apparatus 10B, which is different from that in the processing apparatus 10A. In this case, although the other processing apparatus 10B is not in operation, one of the transfer arms 20A and 20B of the common transfer mechanism 18 holds a wafer that has already undergone alignment. Furthermore, a wafer is also placed in the orientor 16. Consequently, processing of the new wafers placed in the waiting port 4C cannot begin until the current processing executed by the apparatus 10A is complete. In this case, a problem arises in that the rate of operation of the other processing apparatus 10B decreases, thereby hindering improvement in throughput.
In order to solve this problem, it may be adopted that no unprocessed wafers are transferred from the cassette container 6 or placed on standby in the wafer transfer route, until the processing of the current wafer is complete and the wafer is ready for transfer. However, in this case, it takes a long time for an unprocessed wafer to be aligned by the orientor 16, and transferred to a position immediately in front of the processing apparatus 10A. This lowers the throughput of the processing apparatus 10A contrary to the purpose.
As described above, although wafers to be subjected to another process are ready for transfer, the common transfer mechanism 18 and orientor 16 are occupied by wafers to be subjected to the preceding process. Consequently, the wafers to be subjected to the other process cannot be transferred, which lowers the total throughput. Particularly, it is necessary to perform processes of various kinds on a small number of wafers in recent years. Under the circumstances, this problem should be solved as soon as possible.